The structure of compound 48/80, a potent liberator of histamine from mast cells, has been studied by Carbon-13 nuclear magnetic resonance (NMR). Compound 48/80 is a polymer prepared by heating 4-methoxy-N-methylphenethylamine with formaldehyde in acid solution. The NMR spectrum indicates that 48/80 is a polymer in which bridging methylene groups link the 4-methoxy-N-methylphenethylamine in the 3 and 3' positions. The polymer chains are terminated by CH2OH groups. Light scattering measurements indicate that at concentrations above 2 mg/ml compound 48/80 forms micelles. This is reflected in the NMR spectrum by the sharpness of the N-CH3 and the alpha-carbonatom of the phenethylamine chain.